Many sources of energy that can be harvested are in the form of electricity or can be readily converted to it. However in many practical situations, these electricity sources are either weak, with too high or too low a voltage, or with certain other issues. Consequently, they are rarely directly useful to low power electronic devices, for example in wireless sensor networks and Internet of Things (IoT) applications, where a relatively steady supply of a direct current (DC) voltage of a few volts may be required.
It would be advantageous to have systems and methods for efficiently and robustly converting low voltage DC electricity, such as from a thermo-electric generator (TEG), for example a thermopile, into a usable form, for example for use by wireless sensor networks and IoT applications.
An element in a system that turns low voltage DC electricity from a TEG into a usable form is a step-up DC-DC converter. A step-up DC-DC converter converts a low voltage DC input to a higher voltage DC output.
There are various step-up DC-DC converters for TEG energy harvesting available on the market. Many of these schemes rely on an oscillator to do the conversion.
Examples include those proposed in the following three references:    a. EnOcean GmbH, ECT 310 Perpetuum, https://www.enocean.com/en/enocean_modules/ect-310-perpetuum/.    b. Linear Technology Corporation, LTC3108—Ultralow Voltage Step-Up Converter and Power Manager, http://www.linear.com/product/LTC3108.    c. Dario Grgić, Tolgay Ungan, Milos̆ Kostić, and Leonhard M. Reindl, “Ultra-Low Input Voltage DC-DC Converter for Micro Energy Harvesting,” PowerMEMS 2009, pp. 265268, Washington D.C., USA, Dec. 1-4, 2009.
Each of the three schemes introduced above works in a certain input range and at a relatively low efficiency. Their turn-on source conditions and efficiencies are summarized in Table 1 below, where VOC is the open-circuit voltage of the TEG device and RTEG is the internal resistance (also known as the source resistance) of the device. The turn-on source conditions for this example include turn-on voltage and RTEG. The turn-on voltage is the minimum voltage required from the source before the DC-DC converter starts to produce a DC output voltage with an absolute value greater than that of the input.
TABLE 1Turn-on source conditions and efficiencies of existing schemesECT310 byLTC3108 by LinearDario GrgićEnOceanTechnologyet_al.Turn-onVOC = 20 mV &VOC = 50 mV &VOC = 10 mV &sourceRTEG = 2 ΩRTEG = 20 ΩRTEG = 50 Ω1conditionororVOC = 50 mV &VOC = 100 mV &RTEG = 50 ΩRTEG = 50 ΩEfficiency30%20-40%<18%Note that, for Scheme Dario Grgić et al., VOC at RTEG=50Ω is derived from information provided in the abstract of the third reference: Vcc=6 mV (closed-circuit input voltage) and input power of 490 nW.